1. Field of the Invention
The present invention relates to a rare earth metal thin film sensor for detecting hydrogen, and to a differential sensing method for determining the presence of hydrogen in an environment potentially containing same.
2. Description of the Related Art
Hydrogen is a flammable and explosive gas with a wide variety of industrial and scientific uses. Illustrative industrial uses include the production of ammonia, ethanol, methanol, aniline and hydrogen chloride; hydroforming, hydrocracking and hydrorefining of petroleum; hydrogenation of vegetable oils; and reduction of metallic ores.
In space flight applications, H.sub.2 /O.sub.2 mixtures are used in large quantities as propellants for vehicular propulsion systems.
Hydrogen is also used in a variety of metal forming and microelectronic processing steps that rely upon the use of hydrogen of hydrogen containing gases (i.e., forming gases). These processes are often critical in device fabrication and metal interconnect processing of multi-level devices.
In these and other applications, hydrogen sensors are employed to monitor the environment of the process system in which hydrogen is utilized, to safeguard the apparatus and associated operating personnel, as well as to ensure the efficiency and operational integrity of the system. For such purpose, a variety of hydrogen sensors and elaborate detection schemes have been employed and are in common use.
Most of the currently used hydrogen sensors employ sensing arrangements developed several decades ago. A variety of these commercially available hydrogen sensors are based on measuring an electrical characteristic (e.g., resistance) across a sensor element.
The most popular hydrogen sensor is the "catalytic combustible" or "hot wire" sensor (CC sensor).
The CC sensor consists of two beads of resistive elements (Pt/Ir wire) arranged in a Wheatstone bridge configuration and heated to 600-800.degree. C. One bead is coated with a reactive catalyst, while the other is not. Gas is sampled over the beads either by diffusion or by a pump.
In the presence of a flammable gas, the heat of oxidation raises the temperature of the bead and the associated heater element and alters the electrical resistance characteristics of the Wheatstone bridge circuit. This resistance change is related to the concentration of all flammable gases (including H.sub.2) in the vicinity of the sensor.
Unfortunately, in O.sub.2 deficient environments or above the upper explosive limit, the oxidation process is quenched. This causes the heating element of the CC sensor to cool and the needle of the device to fluctuate downward (evidencing a zero or negative value).
Further, since the CC sensor is based upon oxidation, virtually any and all hydrocarbons have the same response.
A further difficulty is that the CC sensor element can be contaminated by halogenated hydrocarbons or poisoned by silicones, lead and phosphorous.
A second commonly used H.sub.2 sensor is a metal oxide semiconductor (MOS) sensor, in which the sensor element consists of mixed Fe, Zn and Sn oxides and is heated to a temperature of 150-350.degree. C. Oxygen atoms absorb on the MOS surface of the sensor element to create an equilibrium concentration of oxide ions in the surface layers.
The baseline resistance (or conductivity) of the MOS sensor in "clean air" is first established by calibration. When certain toxics (e.g., CO or H.sub.2 S) or hydrocarbons come in contact with the sensor, they absorb on the surface of the MOS element. This absorption shifts the oxygen equilibrium, causing a detectable decrease in resistance of the MOS material.
MOS hydrogen sensors have a number of operational deficiencies. They require frequent calibrations (e.g., approximately every 3 months for current commercially available models), exhibit unacceptable response times (3-5 minutes) for rapid detection, can be the source of ignition and fires/explosions, and are incompatible with halogenated vapors.
An important limitation of the above-described commercially available hydrogen sensors is that they are not H.sub.2 specific. All volatile organic compounds (e.g., pentane, toluene, acetone, etc.) as well as gases containing hydrogen (e.g., H.sub.2 S and ethylene) will react with the sensor materials in the sensing elements of these detectors, thereby providing false readings.
A further difficulty with currently available H.sub.2 sensors is that they are often incapable of effectively monitoring process gas streams containing mixtures that include H.sub.2. Such multicomponent gas streams are commonly analyzed by batch gas chromatography and mass spectrometry (GC/MS)--a comparatively expensive and time-consuming process.
These commercially available hydrogen sensors, in addition to being non-discriminating (non-selective) for hydrogen, and susceptible to being easily poisoned, are also bulky, consume a considerable amount of electric power and require frequent calibrations.
Against these many disadvantages and limitations of currently available hydrogen sensors, a major advance is needed in hydrogen sensor technology to overcome such limitations. Among the needed improvements in the field is the provision of H.sub.2 sensors capable of detecting hydrogen in cold and oxygen deficient (helium rich) environments such as in outer space. An H.sub.2 sensor is also needed that eliminates the risk of accidental explosions associated with the use of electric current in presently available sensors. Moreover, a low cost, lightweight, miniature, and hydrogen-specific sensor is highly desirable.
It therefore is an object of the present invention to provide an improved hydrogen sensor and hydrogen sensing methodology overcoming the aforementioned deficiencies of the prior art H.sub.2 detectors.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.